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Enable Near-lossless feature.

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Enable the WebP near-lossless feature by pre-processing the image to smoothen
the pixels.

On a 1000 PNG image corpus, for which WebP lossless (default settings) gets
25% compression gains, following is the performance of near-lossless feature
at various '-near_lossless' levels:
-near_lossless 90: 30% (very very high PSNR 54-60dB)
-near_lossless 75: 38% (very high PSNR 48-54dB)
-near_lossless 50: 45% (high PSNR 42-48dB)
-near_lossless 25: 48% (moderate PSNR 36-42dB)
-near_lossless 10: 50% (PSNR 30-36dB)

WebP near-lossless is specifically useful for discrete-tone images like
line-art, icons etc.

Change-Id: I7d12a2c9362ccd076d09710ea05c85fa64664c38
This commit is contained in:
Vikas Arora
2015-01-29 16:02:09 -08:00
parent c6b24543fc
commit 98c8138663
3 changed files with 7 additions and 19 deletions
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@ -218,12 +218,12 @@ Using this option will discard the alpha channel.
.B \-lossless
Encode the image without any loss.
.TP
.\" .B \-near_lossless " int
.\" Use near-lossless image preprocessing. This option adjusts pixel values
.\" to help compressibility, but has minimal impact on the visual quality.
.\" It triggers lossless compression mode automatically.
.\" Range is 0 (no preprocessing, the default) to 100.
.\" .TP
.BI \-near_lossless " int
Use near-lossless image preprocessing. This option adjusts pixel values
to help compressibility, but has minimal impact on the visual quality.
It triggers lossless compression mode automatically.
Range is 0 (no preprocessing, the default) to 100.
.TP
.BI \-hint " string
Specify the hint about input image type. Possible values are:
\fBphoto\fP, \fBpicture\fP or \fBgraph\fP.